Geoacoustic inversion of ambient noise

Abstract

The vertical directionality of ambient noise is strongly influenced by seabed reflections. Therefore, geoacoustic parameters can be inferred by inversion of the noise. In this approach, using vertical array measurements (16 m aperture), the reflection loss is found directly by comparing the upward with the downward going noise. Theory suggests that this simple ratio is, in fact, the power reflection coefficient (a function of angle and frequency). A layer model and a search is required to find geoacoustic parameters, but no such model or search is required for reflection loss alone. Experimental data have been gathered at 11 sites, 10 in the Mediterranean, and 1 on the New Jersey Shelf during BOUNDARY2001. Site to site variations are discussed, and comparisons are made with simple layer models. Usually the vertical array is bottom-moored, but at two of the Mediterranean sites the array was allowed to drift over a few miles so that bottom properties could be surveyed. Bottom variations could indeed be seen. Simulations using the parabolic equation to mimic spatial variations of bottom properties confirm that the bottom loss inferred from this type of measurement is indeed a local one rather than a regional average.